Light fittings frequently make use of high-power LED technology as the light source. The light yield of an LED is directly related to the current passing through it, a qualitative power supply being necessary for a controlled quantity of light which, for example, is temperature-independent. Control of the light yield is implemented by means of adjustable power supplies or by means of pulse width driving in conjunction with a fixed power supply setting. A number of LEDs can be connected in series and/or parallel from a single power supply, necessitating a higher supply voltage and/or supply current, but avoiding the cost of additional power supply (supplies). Special lighting applications, for example architectural illumination and discotheques, make use of so-called R(ed)G(reen)B(lue) fittings, in recent times often extended to include white LEDs, because of the additional colour dynamics. These RGB and RGBW fittings are used as static illumination with a specific, set colour mix or alternatively as dynamic illumination using lightshow control techniques.
FIG. 1 shows an example of a present-day RGB implementation. For each LED group colour (Red, Blue and Green), this figure shows the pulse width drive switches S1-S3 with associated power supplies PS1-PS3. If the switch S1-S3 of a power supply PS1-PS3 is in the ON position, the power supply PS1-PS3 will send a defined current through the associated LED(s). One characteristic is that each LED has a defined forward voltage drop, which means that each power supply PS1-PS3 is able to drive at most as many LEDs within one circuit as limited by the supply voltage V which is usually subject to the “safe to touch” requirement. The way this works for a light setting is that for a particular brightness and colour setting the pulse width PW1-PW3 for each LED colour is adjusted so as to ensure that said desired setting is achieved.
Simple high-power LED lighting products employ inexpensive linear power supplies, sometimes comprising a simple resistor. At lower light intensity settings, however, a linear power supply leads to inefficient use of energy, owing to heat dissipation in the power supply. Moreover, a dissipating power supply also adds a dissipative loss to the light yield which in any case has already been restricted by a dissipation budget: the LED light from a fitting is limited, inter alia, by the cooling capacity of the fitting. The solution is that energy-efficient products make use of switched power supplies which can have an energy efficiency of more than 90%. Many applications now employ switched power supplies, but there are a number of problems which hinder and even restrict the wider application of this solution.
Energy-efficient switching power supplies consist of a switching voltage source using, as feedback, the measured output current instead of the usual output voltage. The switching power supply and current feedback circuit are relatively bulky owing, in particular, to the use of current-carrying coils, high-frequency capacitors and flyback diodes. The occupied volume, for a 350 mA LED current is about 3 to 4 cm3 per power supply. A present-day solution in the case of RGBW then requires 4 power supplies. The additional volume required by the power supplies, on top of the volume already necessary for the other components of a lighting drive arrangement demands such a large overall volume that as a result at this time the solutions on the market consist of separate modules for the drive arrangement and the fitting. This design restricts a large number of applications requiring more highly integrated solutions such as ceiling lighting, cove lighting and “light-on-a-stick”, with the problems varying from unaesthetic design limitations to simply insufficient installation space for accommodating the drive arrangement.
In the case of separate modules for driving and fitting a distribution of components between drive arrangement and LED groups, such as connection contacts C1-C4 is shown in FIG. 1. Only the LEDs are then still located in the fitting, all the other parts of the drive arrangement being located in a separate module. This results in problems relating to the electromagnetic interference (EMI) of this way of partitioning, owing to the RF modulation of the connecting cables between LEDs and drive arrangement. This results from the high switching currents and associated voltage peaks, which worsen as a function of the cable length. This problem is further aggravated as the LED currents increase with new generations of high-power LEDs. Furthermore, the total LED current of all the groups comes together in a common point C1 (FIG. 1) and associated cable, as a result of which the cable currents and component requirements (e.g. connector) at that single point quadruple in the case of RGBW. Quadrupling of the current, as a consequence of I2R means that power-related requirements are multiplied by a factor of 16.
For an RGB(W) fitting, the large number of power supplies gives rise to an expensive solution resulting from the power supply costs, the multimodule design and associated more expensive cabling. For many applications, these costs constitute a further restriction of the use of LEDs as lighting sources.
From another field of application (display lighting) solutions are known for the low-power LEDs to be driven to be connected in series with one parallel transistor for each LED. If the transistor of an LED is in a conducting state (low-resistance), the current will not pass through the LED (as these demand a higher minimum of voltage to be supplied before an LED current is produced), but only through this transistor: this LED therefore does not give any light. If the transistor is not in a conducting state (high-resistance), virtually all the current will pass through the LED of this transistor, which therefore will give off a quantity of light defined by the power supply.
Concrete examples are described in JP 09081211 and U.S. Pat. No. 4,017,847. As described, all the LEDs are connected in series with transistors parallel thereto. The LEDs connected in series are fed by means of a single power supply. The claimed advantage is a lower total current consumption (at a higher supply voltage). Another example is described in U.S. Pat. No. 4,783,897 and has a similar design: a driven transistor parallel to each LED connected in series. This patent has an additional claim according to which the power supply is not run in the event of all the LEDs being OFF, thereby giving rise to less dissipation for the linear power supply.
As discussed earlier, an energy-efficient, compact and cost-effective solution for driving high-power LEDs for lighting applications demands a switched power supply in conjunction with a pulse width drive arrangement for each LED group. The existing display illumination solutions, however, have in common that they are conceived for single-colour semistatic low-power LED applications such as display background illumination and display indicator illumination. These display illumination solutions cannot, without being modified, solve the problems caused by a dynamic pulse width drive arrangement for high-power multi-LED group lighting owing to the following implementation problems:
The high-power LEDs employ high-intensity currents (0.35-1 amps) in combination with high operating voltages (3.5-4 volts), as a result of which energizing and de-energizing events result in relatively large changes in voltages as a function of time (dV/dt) and relatively large changes in current as a function of time (dI/dt), as a result of, in particular, cable-related parasitic coils and capacitors. Furthermore it is advisable, to avoid dissipation in e.g. a MOSFET switch, to switch the current rapidly in order for the MOSFET to be subject to a high ohmic resistance for as short a time as possible: this, after all, results in I2R dissipation. This effect is further increased if the LEDs are additionally connected in series and/or parallel in one group, since the voltage and/or current required will then increase. Furthermore, a MOSFET switch will also a require a decoupled “floating” drive arrangement which, however, additionally results in a parasitic capacitance between the MOSFET drive arrangement and the LEDs (like the high edge of an H bridge). The use of, for example, 4 LED group drive arrangements in series will therefore lead to a complex current management in the case of an entirely independent pulse width drive arrangement for each group. In principle, each group in the existing display illumination solutions can be actuated at a random instant and it is therefore possible, in the event of e.g. 7 LEDs being actuated simultaneously, for changes in voltage and current in the order of 1 A and 28 V (7 times 4 V) to occur within a few microseconds. This is further aggravated in the case of a switching power supply which in principle will make an additional contribution to the actuation-induced high voltages and current peaks. In the case of the high-power LEDs the display solutions for RGBW will not, owing to the said actuation effects, result in a stable circuit producing qualitatively definable quantities of light per LED group.
As a result of all the current and voltage peaks, available components will often not be able to handle the high current and/or voltage peaks, and additional components would be required to dispel these overdriving problems. These additional components would result in increased volume and cost price, thereby soon negating the advantage of a single power supply.
The pulse width drive principle necessitates the ability to turn a power supply on and off rapidly, to achieve a drive resolution which is sufficient for the desired dynamic light contrast. This resolution leads to relatively high bandwidth requirements for the power supply, at least several tens of kHz. Given a particular implementation, a high-bandwidth switching power supply will still have a limited dV/dt (with respect to component limitations, but also in terms of control stability), and the response rate of a power supply will differ depending on how many LEDs are actuated simultaneously. For example: turning two LEDS on takes e.g. 5 μs, whereas 20 μs are required to turn four LEDs on, since twice the output voltage has to be built up before the desired current is achieved. The fact that there is interaction between the number of LEDs simultaneously actuated and the total effective current through these LEDs (i.e. light yield) results in undesirable cross-links and possibly in oscillation between the drive arrangements of the various LED groups, as a result of which a display solution will not afford a properly operating circuit.
In the event of simultaneous actuation of many LEDs, the supply load against time will also exhibit peaks which pose exacting requirements in terms of cable shielding, as discussed earlier.